What is the best way to get an overdrive auto in a 67 Barracuda?

Here is the math
mph= (rpm x tireD x pi)/(1056x R1R2)
say 58mph =(3600rpmx26xpi)(1056x 1.0Top gear x R2 the diff gear.
Rearranging for R2 we get 1/R2 = (58x1056x1)/(3600x81.7) and then
1/R2=.20824 and finally; R2=4.80 rear gear
Now 4.80 is so far from 3.55 (35%),that one would have to conclude that something is outta whack. And really there is/are only three things it can be; 1) the speedO unit, or 2) the TC is slipping fiercely, or 3) some combination of the two.

As a comparison,my 4-speed car was set up with 3.55s and 26s, way back in '99. And the same formula spits out 2570@56mph, (or 2754rpm at 60mph). And my tach was reading about 2600@56 which is a 1% disagreement.
The math is true.
Since you calibrated your speedO with another car,If I had to guess,I'd guess your TC is slipping big-time

Well there is one other possibility, and that is a failed tranny. But you would have mentioned something about slipping gears.

So at this point, I would be proving that the assumed-to-be 3.55s really are 3.55s,lol. If they are,then I would be calibrating the tach. And if it is good, then I would be pulling that TC. 35% slip is outrageous!

It may be a little early to be shopping for an O/D

At the more typical sub-2400 rpm cruising rpm,the steady-state cruise-rpm will DROP about 150/250 rpm from the accelerating rpms.
For instance if you have a car that cruizes 2200 at 60 mph, run her up to 65 on a good throttle setting, to loc the TC. Pay very close attention to the tach, as you cross 60mph. As you approach 65, back off the throttle to a steady-state cruise, and let the speed drop back to 60mph, and maintain.Now,once the speed has stabilized,re-check the tach.You may find the tach some 150 to 250 rpm lower than the locked rpm. This is typical.
Next still watching the tach, lay on the throttle, and simultaneously apply a bit of brakes. Still at 60mph, you should see the rpms climb some 200plus rpm,rather instantly.This is typical, of TCs under 2800 that I have seen.
What is your TC doing?

I just thought of something.
3600 rpm at 55 is about right for a tranny stuck in second gear.
55x1056x3.55x1.45/(26pi)=3660 rpm. Hmmmmmmmmmmmmmmmmmm

Your thoughts?